“We are concerned that the use of the program on U.S. citizens remains facially unauthorized,” the two wrote.

“We request that DHS stop the expansion of this program and provide Congress with its explicit statutory authority to use and expand a biometric exit program on U.S. citizens,” they wrote.

DHS is currently testing a program to scan the faces of American citizens and foreign nationals in nine cities across the country, including Atlanta, New York and Los Angeles. The program has been carried out voluntarily with airlines.

During a press call after sending the letter, Markey expressed frustration with the DHS approach, saying the agency needs to obtain congressional authorization before it continues testing its face scans.

“When American citizens travel by air internationally, they should not have to choose between privacy and security,” Markey said. “The implementation of the Department of Homeland Security’s facial recognition scanning program for passengers leaving the country raises a number of concerns around accuracy, transparency and basic necessity."

According to numbers provided by the agency, the program would incorrectly flag one in 25 travelers, leading to hundreds of extra stops a day.

The DHS program is opt-in, but Markey says that he would like the DHS to more clearly communicate to travelers that they have the ability to do this.

The agency declined to respond to Markey and Lee’s letter directly, but did say privacy has been a chief consideration.

“[U.S. Customs and Border Protection] takes its privacy obligations seriously and has published Privacy Impact Assessments which clearly state that, for purposes of these initial technical demonstrations, U.S. Citizen participation is not required and alternative procedures are available,” a DHS spokesperson said in a statement emailed to The Hill.

On Thursday, the Center on Privacy and Technology at the Georgetown University Law Center published a study investigating the DHS facial recognition program, in which it similarly concluded that the “DHS’ biometric exit program also stands on shaky legal ground.”

“Congress has repeatedly ordered the collection of biometrics from foreign nationals at the border, but has never clearly authorized the border collection of biometrics from American citizens using face recognition technology," the study said.